Various devices for tightening bolts and checking and measuring the tightening strain therein are currently known in the art and are constituted essentially by spanners for tightening the bolts and ultrasound devices for detecting the tightening strain.
The ultrasound device is constituted by an ultrasound emitter/receiver transducer in which resonance is set up at an ultrasonic frequency, for example, by a quartz crystal excited by a piezoelectric effect; in addition to the transducer, the ultrasound device includes a central unit for processing the signals exchanged with the transducer.
The tightening strain to which a bolt is subjected can easily be found by such a device, which can measure the length of a part or of a bolt provided it is made of homogeneous material, by virtue of the reflection of the elastic waves at the outer surfaces of the part or bolt.
In fact, it is known that, after the bolt has been placed with its head in abutment with the surface, it undergoes an extension proportional to the tightening strain to which is it subjected.
A first technique consists of the use of a spanner separate from the ultrasound device.
Once the bolt has been tightened until its head abuts the surface into which it is being screwed, its length is measured by the ultrasound device and stored as a reference length.
The bolt is then tightened into its seat by the spanner and its length is checked again by the ultrasound device.
The tightening strain in the bolt can easily be found by comparing the new measurement of its length with the reference measurement.
This technique of successive approximations enables a bolt to be tightened with a predetermined tightening strain.
Another currently known technique consists of the use of an automatic screwing device for tightening bolts and measuring the tightening strain, constituted by a screwing head containing an integral ultrasound transducer which can measure the extension of the bolt.
With the use of this latter device, the ultrasound transducer is placed practically in contact with the upper surface of the head of the bolt which is housed in a bush; to ensure an effective coupling between the transducer and the head of the bolt, a film of an ultrasound-permeable fluid, such as, for example, glycerine, grease or a heavy oil, is interposed between the two surfaces.